缓存模块

MyBatis作为一个强大的持久层框架，缓存是其必不可少的功能之一，Mybatis中的缓存分为一级缓存和二级缓存。但本质上是一样的，都是使用Cache接口实现的。缓存位于 org.apache.ibatis.cache包下。

通过结构我们能够发现Cache其实使用到了装饰器模式来实现缓存的处理。首先大家需要先回顾下装饰器模式的相关内容哦。我们先来看看Cache中的基础类的API

// 煎饼加鸡蛋加香肠
“装饰者模式（Decorator Pattern）是指在不改变原有对象的基础之上，将功能附加到对象上，提供了比继承更有弹性的替代方案（扩展原有对象的功能）。”

1. Cache接口

Cache接口是缓存模块中最核心的接口，它定义了所有缓存的基本行为，Cache接口的定义如下:

public interface Cache {/*** 缓存对象的 ID* @return The identifier of this cache*/String getId();/*** 向缓存中添加数据，一般情况下 key是CacheKey  value是查询结果* @param key Can be any object but usually it is a {@link CacheKey}* @param value The result of a select.*/void putObject(Object key, Object value);/*** 根据指定的key，在缓存中查找对应的结果对象* @param key The key* @return The object stored in the cache.*/Object getObject(Object key);/*** As of 3.3.0 this method is only called during a rollback* for any previous value that was missing in the cache.* This lets any blocking cache to release the lock that* may have previously put on the key.* A blocking cache puts a lock when a value is null* and releases it when the value is back again.* This way other threads will wait for the value to be* available instead of hitting the database.*   删除key对应的缓存数据** @param key The key* @return Not used*/Object removeObject(Object key);/*** Clears this cache instance.* 清空缓存*/void clear();/*** Optional. This method is not called by the core.* 缓存的个数。* @return The number of elements stored in the cache (not its capacity).*/int getSize();/*** Optional. As of 3.2.6 this method is no longer called by the core.* <p>* Any locking needed by the cache must be provided internally by the cache provider.*  获取读写锁* @return A ReadWriteLock*/default ReadWriteLock getReadWriteLock() {return null;}}

Cache接口的实现类很多，但是大部分都是装饰器，只有PerpetualCache提供了Cache接口的基本实现。

2. PerpetualCache

PerpetualCache在缓存模块中扮演了ConcreteComponent的角色，其实现比较简单，底层使用HashMap记录缓存项，具体的实现如下：

/*** 在装饰器模式用 用来被装饰的对象* 缓存中的  基本缓存处理的实现* 其实就是一个 HashMap 的基本操作* @author Clinton Begin*/
public class PerpetualCache implements Cache {private final String id; // Cache 对象的唯一标识// 用于记录缓存的Map对象private final Map<Object, Object> cache = new HashMap<>();public PerpetualCache(String id) {this.id = id;}@Overridepublic String getId() {return id;}@Overridepublic int getSize() {return cache.size();}@Overridepublic void putObject(Object key, Object value) {cache.put(key, value);}@Overridepublic Object getObject(Object key) {return cache.get(key);}@Overridepublic Object removeObject(Object key) {return cache.remove(key);}@Overridepublic void clear() {cache.clear();}@Overridepublic boolean equals(Object o) {if (getId() == null) {throw new CacheException("Cache instances require an ID.");}if (this == o) {return true;}if (!(o instanceof Cache)) {return false;}Cache otherCache = (Cache) o;// 只关心IDreturn getId().equals(otherCache.getId());}@Overridepublic int hashCode() {if (getId() == null) {throw new CacheException("Cache instances require an ID.");}// 只关心IDreturn getId().hashCode();}}

然后我们可以来看看cache.decorators包下提供的装饰器。他们都实现了Cache接口。这些装饰器都在PerpetualCache的基础上提供了一些额外的功能，通过多个组合实现一些特殊的需求。

3.BlockingCache

通过名称我们能看出来是一个阻塞同步的缓存，它保证只有一个线程到缓存中查找指定的key对应的数据。

public class BlockingCache implements Cache {private long timeout; // 阻塞超时时长private final